The number of worldwide telephone subscribers is substantial and is rapidly increasing. Each such subscriber is generally connected to a telephone central office via a metallic pair of wires referred to as the telephone "local loop." This huge network of communications paths is grossly under utilized as it is normally used only for sporadic voice communications between telephone subscribers via one or more telephone central offices. In addition, there is a burgeoning demand for data access to telephone subscribers in order to provide such services as interactive television viewing, at-home shopping and banking services, monitoring alarm conditions in the home or office, and a host of other applications in which it is necessary to transmit data to, and receive data from, the telephone subscriber. The known under utilization of the telephone local loop network, and the growing demand for access to the telephone subscriber, has sparked a search for means to utilize the telephone local loop for information transfer, while leaving the underlying voice communications undisturbed.
Systems now exist which permit the simultaneous transmission of both voice and a secondary information signal over the local loop. One such system is described in U.S. Pat. Nos. 4,493,948 and 4,528,422, assigned to The Inteleplex Corporation, the present assignee of the instant invention. Such systems generally comprise an information terminal coupled to the ring and tip conductors of the local loop at a first location, such as the location of the telephone subscriber, and a companion information terminal also coupled to the ring and tip conductors at a second location, such as the telephone central office. The information terminals are designed to transmit complementary signals, in both amplitude and phase, on the ring and tip conductors between a respective conductor and earth ground. These signals are carefully balanced longitudinally between the tip and ring conductors and, when precisely balanced, the signals will not disturb the normal underlying voice communications. It is, of course, understood that voice communications is not disturbed because the telephone equipment works on the principal of detecting a difference in voltage and/or phase between the signals on the tip and ring conductors. As a result, perfectly balanced signals are essentially invisible to the telephone equipment.
Systems like those described in the above-referenced two U.S. patents provide excellent service in permitting a secondary information signal to be superimposed over normal voice communications carried on the local loop. However, in order for such systems to operate properly, perfect signal balance must be maintained between the tip and ring conductors at all times.
A problem arises in maintaining balanced signals when transmitting over metallic tip and ring conductors of the local loop. Factors such as differences in tip and ring conductor lengths, differences in manufacturing tolerances, and various environmental conditions can change the impedance of either of the conductors. Any change in impedance, even a very small one, results in unbalanced longitudinal signals, i.e., differences exist between amplitude and phase of such signals. Such differences, in turn, result in unwanted interference (cross-talk) with voice communications being carried over the local loop.
Previous solutions to this problem have included the use of manually adjustable potentiometers within the local loop that require periodic adjustment by a technician. However, the maintenance expenses alone associated with a circuit requiring such manual adjustment would render it impractical to use this technique widely, such as in conjunction with the existing telephone network.
Most recently, in a pending application entitled "Automatic Balancing Circuit for Longitudinal Transmission System", apparatus is disclosed which automatically provides phase and amplitude balancing of two signals coupled on the local loop. One signal is carried between a ring conductor and ground while the second is carried between a tip conductor and ground. While this technique allows two signals to be simultaneously coupled on the local loop at the same time, transient signals occur on the loop which interfere with such simultaneous transmission despite the fact that the two signals are balanced. One such transient occurs during the on-hook to off-hook signal supplied by a subscriber telephone to a central office while the other appears at varying times during the duration of the ringing signal coupled from the central office to the subscriber telephone. It would, therefore, be extremely desirable to eliminate the information loss due to the existence of such transients in a manner which is compatible with the operation of the existing voice communications network and which permits simultaneous use of the local loop for voice communications and another information signal.